1. Field of the Invention
This invention relates to a real image mode finder optical system suitable for use in a lens shutter camera or an electronic still camera which is constructed so that a photographing optical system is independent of a finder optical system, and in particular, to a real image mode finder optical system which has a large angle of emergence and is best adapted for mounting to a compact camera.
2. Description of Related Art
In general, finders constructed to be independent of photographing optical systems, used in lens shutter cameras, are roughly divided into two classes: virtual image mode finders and real image mode finders.
The virtual image mode finder has the advantage that an image erecting optical system is not required, but has the disadvantage that since an entrance pupil is located at the same position as an observer""s pupil, the diameter of a front lens must be increased or the area of a visual field is not defined. An Albada finder of this type allows the area of the visual field to be definitely set, but has the problem that a half mirror coating is applied to the surface of a lens and thus the transmittance of the lens is reduced or flare is increased.
In contrast to this, the real image mode finder is such that the position of the entrance pupil can be located on the object side, and hence the diameter of the front lens can be decreased. Moreover, by placing a field frame in the proximity of the imaging position of an objective lens, the area of the visual field can be defined without reducing the transmittance.
A conventional real image mode finder, however, dose not provide a sufficient angle of apparent visual field (hereinafter referred to as an angle of emergence). Specifically, an object to be observed can be viewed only in small size. Thus, when the object is a person, there is the problem that it is difficult to view the expression of the person. A finder with a relatively large angle of emergence is disclosed, for example, in each of Japanese Patent Preliminary Publication Nos. Hei 6-51201 and Hei 11-242167. However, even such a finder does not provide a sufficiently large angle of emergence.
A so-called telescope has a large angle of emergence. However, the telescope, which has a high magnification, namely a small angle of visual field, cannot be applied to a finder constructed to be independent of the photographing optical system, used in a common lens shutter camera which has a wide angle of view.
It is, therefore, an object of the present invention to provide a real image mode finder optical system in which the angle of emergence can be increased and compactness can be attained.
In order to achieve this object, the real image mode finder optical system according to the present invention is constructed to be independent of the photographing optical system and has, in order from the object side, an objective optical system with a positive refracting power, a field frame located in the proximity of the imaging position of the objective optical system, and an eyepiece optical system with a positive refracting power. The real image mode finder optical system includes an image erecting means, and the focal length of the objective optical system can be made shorter than that of the eyepiece optical system. In this case, the real image mode finder optical system satisfies the following condition:
0.52 less than mh/fe less than 1xe2x80x83xe2x80x83(1) 
where mh is the maximum width of the field frame and fe is the focal length of the eyepiece optical system.
The real image mode finder optical system according to the present invention is constructed to be independent of the photographing optical system and has, in order from the object side, an objective optical system with a positive refracting power, a field frame located in the proximity of the imaging position of the objective optical system, and an eyepiece optical system with a positive refracting power. The real image mode finder optical system includes an image erecting means, the objective optical system includes three of reflecting surfaces of the image erecting means, and the eyepiece optical system includes one of reflecting surfaces of the image erecting means so that an image is erected through four reflecting surfaces comprised of three reflecting surfaces of the objective optical system and one reflecting surface of the eyepiece optical system. The focal length of the objective optical system is variable, and when the magnification of the finder optical system is changed, at least two lens units are moved along different paths. The focal length of the objective optical system at the wide-angle position thereof is shorter than that of the eyepiece optical system. In this case, the real image mode finder optical system satisfies Condition (1).
The real image mode finder optical system according to the present invention is constructed to be independent of the photographing optical system and has, in order from the object side, an objective optical system with a positive refracting power, a field frame located in the proximity of the imaging position of the objective optical system, and an eyepiece optical system with a positive refracting power. The real image mode finder optical system includes an image erecting means, the objective optical system includes three of reflecting surfaces of the image erecting means, and the eyepiece optical system includes one of reflecting surfaces of the image erecting means so that an image is erected through four reflecting surfaces comprised of three reflecting surfaces of the objective optical system and one reflecting surface of the eyepiece optical system. The focal length of the objective optical system is variable, and when the magnification of the finder optical system is changed, at least two lens units are moved along different paths. The focal length of the objective optical system at the wide-angle position thereof is shorter than that of the eyepiece optical system. The image erecting means including the three reflecting surfaces is constructed with two prisms so that each of the prisms has at least one reflecting surface and one of the entrance surface and the exit surface of each prism is configured as a curved surface with finite curvature.
The real image mode finder optical system according to the present invention is constructed to be independent of the photographing optical system and has, in order from the object side, an objective optical system with a positive refracting power, a field frame located in the proximity of the imaging position of the objective optical system, and an eyepiece optical system with a positive refracting power. The objective optical system has an image erecting means including four reflecting surfaces. The focal length of the objective optical system is variable, and when the magnification of the finder optical system is changed, at least two lens units are moved along different paths. The focal length of the objective optical system at the wide-angle position thereof is shorter than that of the eyepiece optical system. In this case, the real image mode finder optical system satisfies Condition (1).
The real image mode finder optical system according to the present invention is constructed to be independent of the photographing optical system and has, in order from the object side, an objective optical system with a positive refracting power, a field frame located in the proximity of the imaging position of the objective optical system, and an eyepiece optical system with a positive refracting power. The objective optical system has an image erecting means including four reflecting surfaces. The focal length of the objective optical system is variable, and when the magnification of the finder optical system is changed, at least two lens units are moved along different paths. The focal length of the objective optical system at the wide-angle position thereof is shorter than that of the eyepiece optical system.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.